Fibrous composites are promising for making ceramics considerably tougher by imparting resistance to crack propagation to the ceramic. Carbon and silicon carbide fiber reinforcement strengthens the brittle matrices of glasses and glass ceramics. A problem exists with oxide-based fibers, however, because they degrade or react with the matrix when the ceramics or glasses are molded. The reaction between the fiber and the matrix results in a high degree of bonding which renders the toughening mechanisms inoperable. This problem for oxide-based fibers can be solved by creating a barrier coating on the fibers. Preferably, a suitable barrier coating would be relatively inert, and would comprise a physical separation between the ceramic or glass matrix and the reinforcing fiber. The barrier would eliminate any reactions between the oxide fiber and the ceramic matrix or would substantially slow the kinetics of such reaction so that toughening would result in the composite.
While there are many methods for creating a boron nitride coating on fibers, none is completely suitable. For example, a boron nitride coating can be formed by exposing the fibers to boron oxide or boric acid. Boron oxide, an intermediate in the process, is extremely damaging to oxide-based ceramic fibers, so the process cannot be used to barrier coat them. Chemical vapor deposition (CVD) has been used to provide boron nitride coatings. Chemical vapor deposition is unsatisfactory because it requires the careful and precise injection of predetermined amounts of reactive gases containing boron and nitrogen and precise control of the temperature. Only then can the gases react at or near the surface of the fiber and deposit a coating on the surface. Control of the deposition thickness and of the quality of the barrier coating is difficult. The most serious difficulty with the CVD process, however, is ensuring that the coating is uniform about the entire surface of the fiber. In the CVD process, the gaseous components react on the first hot surface on which they come into contact. Thus, CVD coating is usually limited to monofilament applications where the entire fiber surface is readily accessible. Fiber cloths or yarns are not as amenable to creation of uniform coatings through the CVD process.